The present invention relates to a conductive resin that functions as an adhesive that is used for electric connections in an electronic component or as a heat transfer medium in an electronic component, an electronic-component-mounted body in which the conductive resin is used, and a method of manufacturing an electronic-component-mounted body.
Due to recent increased ecological awareness, in the field of the electronic component mounting techniques, limitations are about to be imposed on lead contained in a solder. Hence, it is an urgent task to establish a junction technique that does not utilize lead in the mounting of electronic components. Examples of the lead-free mounting technique include a technique in which a lead-free solder is used and a technique in which a conductive resin is used. In particular, increased attention is directed to the conductive resin, which is expected to achieve such advantages as: providing increased flexibility of a junction portion; and allowing the mounting process to be conducted at a lower temperature, to be organic-solvent-free, and not to require washing.
A conventional conductive resin typically is a resin obtained by dispersing a conductive filler such as a metallic powder in an epoxy resin-based binder resin component. For instance, in the case where a component electrode of an electronic component is connected with a substrate electrode of a circuit substrate using a conductive resin, contact can be obtained by means of the binder resin between respective particles of the conductive filler, between the conductive filler and the component electrode, and between the conductive filler and the substrate electrode, hence electric connection is provided therebetween, respectively. At the same time, the electronic component and the substrate electrode are bonded with each other with the conductive resin, thereby being mechanically connected with each other.
In the case of a mounting technique utilizing a conductive resin, the connection at a junction portion between the electronic component and the circuit substrate is provided via the resin component, which is flexibly deformed in response to heat or external force. Thus, the resin has an advantage in that cracks hardly occur as compared with a solder that provides a junction portion made of an alloy. Therefore, great expectations are placed on the resin as a substitute for the solder.
However, in an attempt to use a conventional conductive resin as a substitute for the solder, it has been difficult to achieve connection strength substantially equal to that of the solder.
A property of the conductive resin for bonding the component electrode with the substrate electrode is exhibited, as described above, when, for instance, an epoxy resin-based binder resin adheres to the component electrode and to the substrate electrode. The epoxy resin-based binder resin is one of the resin materials having the greatest bond strength with metals, and is considerably superior to the other resin materials in mechanical strength of the resin per se after being cured. Therefore, it is used in various adhesive structural members. It, however, does not provide an alloy bonding like at a solder junction portion, and hence, it is difficult to achieve a junction strength substantially equal to that achieved by the solder against an external force applied to a junction portion, such as shearing, pulling, bending, and twisting.
Principal causes of this problem are described below, with reference to FIG. 6.
FIG. 6 is a schematic cross-sectional view of an electronic-component-mounted body (hereinafter sometimes referred to as mounted body) in which an electrode 45 of an electronic component 44 is connected with a substrate electrode 42 of a circuit substrate 41 using a conventional conductive resin 43. The conductive resin 43 is disposed at a predetermined position on the substrate electrode 42, for instance, by printing or by means of a dispenser. Thereafter, the electronic component 44 is mounted thereon and the resin is cured with heat, so that electric connection and mechanical connection are provided. Here, a junction portion obtained maintains a size upon the aforementioned printing. Besides, throughout a junction interface between the electrode 45 and the conductive resin 43 and a junction interface between the substrate electrode 42 and the conductive resin 43, a conductive filler and a binder resin are dispersed and mixed with each other substantially homogeneously.
Generally the content of the conductive filler in the conventional conductive resin is approximately 80 percent by weight. The conductive filler, made of silver for instance, has a specific gravity of approximately 10, and the binder resin has a specific gravity of approximately 1.1. Therefore, an actual contact area of the binder resin with the component electrode and the substrate electrode, which actually provides the mechanical connection at the foregoing junction portion, that is, the junction strength, is approximately xc2xd of an apparent junction area. Therefore, the junction strength lowers as compared with the case of the binder resin solely.
In other words, in spite of the advantages of flexibility and rare occurrence of cracks in comparison with those of the solder, the conventional conductive resin has a disadvantage of insufficient absolute junction strength as compared with that of the solder. Consequently, the conductive resin has not yet been widely used as an alternative junction material for the solder.
An object of the present invention is to provide a new conductive resin that maintains the advantage of hardly cracking and that provides an increased junction strength relative to a conventional conductive resin, an electronic-component-mounted body having an increased junction strength, and a manufacturing method of the same.
To achieve the foregoing object, the present invention provides a conductive resin that is used to bond an electronic component and a substrate electrode, and upon being heated, comes to have a portion containing a conductive filler as a main component and a portion containing a binder resin as a main component. Further, the present invention provides an electronic-component-mounted body in which a portion containing a conductive filler as a main component and a portion containing a binder resin as a main component are formed, in a conductive resin. Further, the present invention provides a method for manufacturing an electronic-component-mounted body in which a portion containing a conductive filler as a main component and a portion containing a binder resin as a main component are substantially separately formed, in a conductive resin.
More specifically, a conductive resin of the present invention includes a conductive filler and a binder resin, the conductive filler and the binder resin constituting a main component of the conductive resin, in which the conductive filler is dispersed substantially homogeneously in the binder resin. The conductive resin is characterized in that the conductive resin before being cured is used to bond a plurality of adhesion targets, and when the conductive resin is heated, the conductive filler and the binder resin are substantially separated from each other, so that the conductive resin after being cured has a portion containing the conductive filler as a main component and a portion containing the binder resin as a main component. This configuration thus allows a portion having the binder resin as the main component to be formed in the cured conductive resin, and hence, makes it possible to increase the junction strength between the adhesion targets by means of this portion.
Furthermore, an electronic-component-mounted body of the present invention includes an electronic component having an electrode and a circuit substrate having an electrode, in which the electrode of the electronic component and the electrode of the circuit substrate are connected with each other via a conductive resin containing a conductive filler and a binder resin. The electronic-component-mounted body is characterized in that, in a region where the electrodes are opposed to each other, the conductive resin has the conductive filler as a main component, and that, in a region except for the region where the electrodes are opposed to each other, at least a part of the conductive resin has the binder resin as a main component. In this configuration, the portion of the conductive resin having the conductive filler as the main component provides electric connection and mechanical connection, while the portion thereof having the binder resin as the main component provides electric insulation and mechanical connection. The presence of the portion having the binder resin as the main component particularly allows the junction strength of the electronic component to increase.
Furthermore, a first electronic-component-mounted body manufacturing method of the present invention is characterized by including the steps of applying the foregoing conductive resin of the present invention at a predetermined position on a circuit substrate; placing an electronic component at the predetermined position where the conductive resin is applied; and curing the conductive resin. These steps are conducted in the stated order.
Furthermore, a second electronic-component-mounted body manufacturing method of the present invention is characterized by including the steps of: applying to at least either one of a circuit substrate and an electronic component a material effective in improving the wettability of a binder resin of a conductive resin with respect to the circuit substrate or a component member of the electronic component; applying the conductive resin at a predetermined position on the circuit substrate; placing the electronic component at the predetermined position where the conductive resin is applied; and curing the conductive resin. These steps are conducted in the stated order.
Furthermore, a third electronic-component-mounted body manufacturing method of the present invention is characterized by comprising the steps of: applying a conductive resin at a predetermined position on a circuit substrate; applying to at least one of the circuit substrate, the conductive resin, and an electronic component a material effective in improving wettability of a binder resin of the conductive resin with respect to the circuit substrate or a component member of the electronic component; placing the electronic component at the predetermined position where the conductive resin is applied; and curing the conductive resin. These steps are conducted in the stated order.
Furthermore, a fourth electronic-component-mounted body manufacturing method is characterized by including the steps of: applying a conductive resin at a predetermined position on a circuit substrate; placing an electronic component at the predetermined position where the conductive resin is applied; applying to at least a portion where an electronic component is placed a material effective in improving wettability of a binder resin of the conductive resin with respect to the circuit substrate or a component member of the electronic component; and curing the conductive resin. These steps are conducted in the stated order.
According to the first electronic-component-mounted body manufacturing method, the conductive resin of the present invention is used. Therefore, the conductive filler and the binder resin in the conductive resin substantially are separated when being heated, and hence, a portion having the conductive filler as the main component and a portion having the binder resin as the main component are formed in the cured conductive resin.
Furthermore, according to the second to fourth electronic-component-mounted body manufacturing method, the material effective in improving the wettability of the binder resin of the conductive resin with respect to the circuit substrate or the electronic component is applied. Therefore, the conductive filler and the binder resin substantially are separated in a subsequent heating step, and a portion having the conductive filler as the main component and a portion having the binder resin as the main component are formed in the cured conductive resin.
Consequently, any one of the first through fourth mounted body manufacturing methods makes it possible to increase the junction strength of the electronic component by means of the portion having the binder resin as the main component.